New lithosphere is constantly being produced at the oceanic ridges. However, our planet is not growing larger; its total surface area remains constant. A balance is maintained because older, denser portions of oceanic lithosphere descend into the mantle at a rate equal to seafloor production. This activity occurs along convergent plate boundaries, where two plates move toward each other and the leading edge of one is bent downward as it slides beneath the other. Convergent plate boundaries are thought of as destructive plate margins because oceanic crust is destroyed where they occur.

Convergent boundaries are also called subduction zones because they are sites where lithosphere is descending (being subducted) into the mantle. Lithospheric plates subduct (sink) because their density is greater than the density of the underlying asthenosphere. (Recall that oceanic crust has a greater density than continental crust because oceanic crust is largely composed of dense ferromagnesian-rich mineral.) In general, old oceanic lithosphere is about 2 percent more dense than the underlying asthenosphere, and it sinks much like an anchor on a ship. Continental lithosphere, in contrast, is less dense than the underlying asthenosphere and tends to resist subduction. However, there are a few locations where continental lithosphere is thought to have been forced below an overriding plate, albeit to relatively shallow depths.

Deep-ocean trenches are long, linear depressions in the seafloor that are generally located only a few hundred kilometers offshore of either a continent or a chain of volcanic islands, such as the Aleutian chain. These underwater surface features are produced where oceanic lithosphere bends as it descends into the mantle along subduction zones (refer to Figure 15A). An example is the Peru–Chile trench, located along the west coast of South America. It is more than 4500 kilometers (3000 miles) long, and its floor is as much as 8 kilometers (5 miles) below sea level. Western Pacific trenches, including the Mariana and Tonga trenches, are even deeper than those of the eastern Pacific.

Slabs of oceanic lithosphere descend into the mantle at angles that vary from a few degrees to nearly vertical (90 degrees). The angle at which oceanic lithosphere subducts depends largely on its age and, therefore, its density. For example, when seafloor spreading occurs relatively near a subduction zone, as is the case along the coast of Chile (refer to Figure 10B), the subducting lithosphere is young and buoyant, which results in a low angle of descent. As the two plates converge, the overriding plate scrapes over the top of the subducting plate below—a type of forced subduction. Consequently, the region around the Peru–Chile trench experiences great earthquakes, including the 2010 Chile earthquake—one of the 10 largest quakes in recorded history.

As oceanic lithosphere ages (moves farther from the spreading center), it gradually cools, which causes it to thicken and increase in density. In parts of the western Pacific, some oceanic lithosphere is 180 million years old—the thickest and densest in today’s oceans. The very dense slabs in this region typically plunge into the mantle at angles approaching  degrees. This largely explains why most trenches in the western Pacific are deeper than trenches in the eastern Pacific.

Although all convergent zones have the same basic characteristics, they may vary considerably depending on the type of crustal material involved and the tectonic setting. Convergent boundaries can form between one oceanic plate and one continental plate, between two oceanic plates, or between two continental plates (Figure 15).

Oceanic–Continental Convergence

When the leading edge of a plate containing continental crust converges with a slab of oceanic lithosphere, the buoyant continental block remains “floating,” while the denser oceanic slab sinks into the mantle (refer to Figure 15A). When a subducting oceanic slab in oceanic-continental convergence reaches a depth of about 100 kilometers (60 miles), melting is triggered within the wedge of hot asthenosphere that lies above it. But how does the subduction of a cool slab of oceanic lithosphere cause mantle rock to melt? The answer lies in the fact that water contained in the descending plates acts the way salt does to melt ice. That is, “wet” rock in a high-pressure environment melts at substantially lower temperatures than does “dry” rock of the same composition.

Sediments and oceanic crust contain large amounts of water, which is carried to great depths by a subducting plate. As the plate plunges downward, heat and pressure drive out water from the hydrated (water-rich) minerals in the subducting slab. At a depth of roughly 100 kilometers (60 miles), the wedge of mantle rock is sufficiently hot that the introduction of water from the slab below leads to some melting. This process, called partial melting, is thought to generate some molten material, which is mixed with unmelted mantle rock. Being less dense than the surrounding mantle, this hot mobile material gradually rises toward the surface. Depending on the environment, these mantle-derived masses of molten rock may ascend through the crust and give rise to a volcanic eruption. However, much of this material never reaches the surface but solidifies at depth—a process that thickens the crust.

The volcanoes of the towering Andes were produced by molten rock generated by the subduction of the Nazca plate beneath the South American continent (refer to Figure 10). Mountain systems like the Andes, which are produced in part by volcanic activity associated with the subduction of oceanic lithosphere, are called continental volcanic arcs. The Cascade Range in Washington, Oregon, and California is another mountain system consisting of several well-known volcanoes, including Mount Rainier, Mount Shasta, Mount St. Helens, and Mount Hood (Figure 16). This active continental volcanic arc also extends into Canada, where it includes Mount Garibaldi and Mount Meager.

Oceanic–Oceanic Convergence

An oceanic–oceanic convergent boundary has many features in common with oceanic–continental plate margins (refer to Figure 15A,B). Where two oceanic slabs converge, one descends beneath the other, initiating volcanic activity by the same mechanism that operates at all subduction zones (refer to Figure 10). Water released from the subducting slab of oceanic lithosphere triggers melting in the hot wedge of mantle rock above. In this setting, volcanoes grow up from the ocean floor rather than upon a continental platform. Sustained subduction eventually results in a chain of volcanic structures large enough to emerge as islands. The newly formed land, consisting of an arc-shaped chain of volcanic islands, is called a volcanic island arc, or simply an island arc (Figure 17).

The Aleutian, Mariana, and Tonga Islands are examples of relatively young volcanic island arcs. Island arcs are generally located 120 to 360 kilometers (75 to 225 miles) from a deep-ocean trench. Located adjacent to the island arcs just mentioned are the Aleutian trench, the Mariana trench, and the Tonga trench.

Most volcanic island arcs are located in the western Pacific. Only two are located in the Atlantic—the Lesser Antilles arc, on the eastern margin of the Caribbean Sea, and the Sandwich Islands, located off the tip of South America. The Lesser Antilles are a product of the subduction of the Atlantic seafloor beneath the Caribbean plate. Located within this volcanic arc are the Virgin Islands of the United States and Britain as well as Martinique, where Mount Pelée erupted in 1902, destroying the town of St. Pierre and killing an estimated 28,000 people. This chain of islands also includes Montserrat, where volcanic activity has occurred as recently as 2013.

Island arcs are typically simple structures made of numerous volcanic cones underlain by oceanic crust that is generally less than 20 kilometers (12 miles) thick. Some island arcs, however, are more complex and are underlain by highly deformed crust that may reach 35 kilometers (22 miles) in thickness. Examples include Japan, Indonesia, and the Alaskan Peninsula. These island arcs are built on material generated by earlier episodes of subduction or on small slivers of continental crust that have rafted away from the mainland.

Continental–Continental Convergence

The continental-continental convergence type of convergent boundary results when one landmass moves toward the margin of another because of subduction of the intervening seafloor (Figure 18A). Whereas oceanic lithosphere tends to be dense and readily sinks into the mantle, the buoyancy of continental material generally inhibits it from being subducted, at least to any great depth. Consequently, a collision between two converging continental fragments ensues (Figure 18B). This process folds and deforms the accumulation of sediments and sedimentary rocks along the continental margins as if they had been placed in a gigantic vise. The result is the formation of a new mountain belt composed of deformed sedimentary and metamorphic rocks that often contain slivers of oceanic lithosphere.

Such a collision began about  million years ago, when the subcontinent of India “rammed” into Asia, producing the Himalayas—the tallest mountain range on Earth that includes the 8848.86-meter (29,031.69-foot) Mount Everest (Figure 18C). During this collision, the continental crust buckled and fractured and was generally shortened horizontally and thickened vertically. In addition to the Himalayas, several other major mountain systems, including the Alps, Appalachians, and Urals, formed as continental fragments collided.

• When oceanic plates move toward one another, oceanic lithosphere is subducted into the mantle, where it is recycled. Subduction manifests itself on the ocean floor as a deep linear trench. The subducting slab of oceanic lithosphere can descend at a variety of angles, from nearly horizontal to nearly vertical.

• Aided by the presence of water, the subducted oceanic lithosphere triggers melting in the mantle, which produces magma. The magma is less dense than the surrounding rock and will rise. It may cool at depth, thickening the crust, or it may make it all the way to Earth’s surface, where it erupts as a volcano.

• A line of volcanoes that emerge through continental crust is termed a continental volcanic arc, while a line of volcanoes that emerge through an overriding plate of oceanic lithosphere is called a volcanic island arc.

• Continental crust resists subduction due to its relatively low density, when an intervening ocean basin is completely destroyed through subduction, the continents on either side collide, generating a new mountain range.

• convergent plate boundaries: Region where two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle. It can also involve the collision of two continental plates to create a mountain system. When referring to a boundary where oceanic lithosphere is pushed beneath the overriding place, it is also called a subduction zone.

• convergent volcanic arcs: Mountain systems formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.

• partial melting: The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.

• subduction zones: Convergent plate boundaries in which two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle. Also called a convergent plate boundaries.

• volcanic island arc (a.k.a., island arc): An arc-shaped chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of oceanic lithosphere beneath another slab of oceanic lithosphere is occurring. Also called simply an island arc.